Historic amendments to 300-year-old laws which give male heirs priority will be backdated, ministers announced over the summer.

That means that a daughter born to the happy couple would not be outranked by a younger brother under the outdated succession laws which were set out in the 1701 Act of Settlement.

There had been fears of a constitutional crisis if William and Kate had a child before the reforms were passed by all 16 countries where the Queen is head of State.

The current law of male primogeniture only allows Elizabeth II to be Wueen because she did not have any brothers.

It also means that the Princess Royal, the Queen’s second born, is just tenth in line to the throne behind her younger brothers Andrew (fourth) and Edward (seventh).

Courtiers had feared that trying to reform the law would open an Pandora’s Box, with republicans in Australia threatening to use the exercise to oust the Queen.

But Her Majesty backed the changes at a Commonwealth gathering in Australia last year when she urged political leaders to “find ways to allow all girls and women” to play their part in national life.

David Cameron also told other Commonwealth leaders that in an age of “gender equality” the 1701 laws was out of date and discriminatory.

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All 16 countries voted for changes that will also lift the ban on anyone married to a Roman Catholic from taking the throne.

And an ancient and unused rule saying descendants of George II are supposed to gain the consent of the monarch to marry will only apply to the first six people in the line of succession.

The change needs to be legislated for in the Commonwealth nations of the UK, Australia, New Zealand, Canada, Jamaica, Antigua and Barbuda, the Bahamas, Barbados, Grenada, Belize, St Christopher and Nevis, St Lucia, the Solomon Islands, Tuvalu, St Vincent and the Grenadines and Papua New Guinea.

Deputy Prime Minister Nick Clegg told the Commons in May: “If the birds and bees were to deliver that blessing to the Duke and Duchess of Cambridge, and indeed the nation, then that little girl would be covered by the provisions of these changes of the rules of succession because they operate from the time of the declaration of the Commonwealth summit last October.

“It is very important to remember that the rules are de facto in place, even though they have yet to be implemented through legislation in the way that I have described.”

Britain would now be ruled by Carl von Wied, a 50-year-old German father-of-three, if girls had been allowed to inherit the throne ahead of younger brothers.